1. Field of the Disclosure
The present disclosure relates to a multi stage safety injection device using a flow control line capable of injecting coolant into a reactor vessel step by step when an accident occurs in a reactor, and a passive safety injection system having the same.
2. Description of the Related Art
A Reactor can be classified according to the configuration of a safety system or the installation location of a main component. According to the characteristics of a safety system, a reactor can be classified into i) an active reactor using an active force such as a pump or the like, and ii) a passive reactor using a passive force such as gravity force, pressure force or the like. And also according to the installation location of main components(steam generators, a pressurizer, and reactor coolant pumps), a reactor can be classified into i) a loop type reactor in which main component are installed out of the reactor vessel, and ii) an integral reactor in which main components are installed within the reactor vessel.
When an accident occurs in a reactor, passive tanks with various types are used to supply emergency cooling water to a reactor vessel. i) A nitrogen pressurized safety injection tank (accumulator) for rapidly supplying coolant to a reactor during a large break loss of coolant accident, in which a large line is fractured to outflow a large amount of coolant, is used in many commercial loop type water reactors, and ii) a core makeup tank using a gravitational head of water subsequent to making a pressure balance between the reactor and tank is used in addition to a nitrogen pressurized safety injection tank in the U.S. Westinghouse passive loop type reactors such as AP600, AP1000 and so on.
In the integral reactor, main components such pumps and steam generators or the like are installed within the reactor vessel contrary to the commercial loop type water reactor, and thus there are no large lines for connecting the main components. Accordingly, the lines, which connecting a reactor vessel and systems such as a chemical and volume system, a safety injection system, a shutdown cooling system, a safety valve, and the like, have small size in an integral reactor. Due to these characteristics, a large break loss of coolant accident, where large lines are fractured, is eliminated fundamentally.
Furthermore, in the integral reactor (SMART reactor: Korea, mPower: B&W Company, NuScale: NuScale Power LLC, WEC-SMR: Westinghouse Electric Company), main components are installed in a reactor vessel, where a large amount of coolant exists. Accordingly, when an accident, a loss of coolant accident due to a break such as a line fracture or the like, occurs in the integral reactor, the pressure and water level within the reactor vessel are slowly decreased compared to those of a loop type reactor.
A passive safety system in a reactor refers to a system for safely maintaining a reactor only using a natural force contained in the system without using an active device such as a pump even without operator's action or safety rated AC power such as an emergency diesel generator for more than a period of time (currently required 72 hours) required for the reactor when an accident occurs.
Even the integral reactor has such characteristics, in general, the integral reactor requires i) a high flow rate of safety injection at the initial stage of the accident in which the core level is relatively fast decreased, ii) a medium-level amount of (medium flow rate of) safety injection at the early and middle stages of the accident in which the coolant discharge flow rate of the reactor is relatively large due to a high pressure of the reactor vessel, and iii) a relatively small amount of (low flow rate of) safety injection at the middle and late stages of the accident in which the coolant discharge flow rate is greatly reduced due to a decreased pressure of the reactor vessel. It is noted that a high flow rate of the integral reactor is quite smaller compared to a flow rate required in the commercial loop type reactor.
Although the absolute amount of safety injection is greatly different from the integral reactor, a loop type reactor in the related art also requires various safety injection characteristics from the case of rapidly supplying a lot of fluid at the initial stage of an accident to the case of continuously supplying a small amount of fluid at the last stage of an accident. A nitrogen pressurized safety injection tank in the related art is typically designed to quickly inject a high flow rate of coolant in a safe manner when the internal pressure of the reactor vessel is rapidly decreased, and thus mostly used as a facility for coping with a large break loss of coolant accident. Furthermore, a core makeup tank in the related art is designed to implement safety injection with one injection characteristic due to a gravitational head of water subsequent to making a pressure balance between the reactor vessel and the core makeup tank, and mostly used for the purpose of injecting emergency boric acid solution at a relatively high pressure when a non-loss of coolant accident occurs. As described above, various safety injection characteristics cannot be satisfied with a single type of safety tank, and thus various type systems are used in a complicated manner to meet the required characteristics of safety injection in an individual reactor during an accident.
For instance, i) a pressure balance core makeup tank (safety injection at high pressure), a pressurized safety injection tank (safety injection at medium pressure), an in-containment refuelling water storage tank (safety injection at low pressure), and the like are used in a complicated manner in a passive safety system of the passive pressurized water reactors such as AP600, AP1000 and the like, and ii) a pressurized safety injection tank (safety injection at medium pressure), a high-pressure safety injection pump, a low-pressure safety injection pump, and the like are used in a complicated manner in an active safety system.
Accordingly, it is required to develop a device for simplifying safety injection facilities that have been configured in a complicated manner according to the required characteristic of safety injection into the reactor to effectively inject coolant.